The Future of Fuel Efficiency

Suck. Squeeze. Bang. Blow. There’s no joke to be made there—you’re looking at the DNA of the four-stroke internal combustion engine, virtually unchanged since Dr. Nikolaus Otto first built it in Germany back in 1876.

Despite this system’s longevity, we’re still finding ways to make engines more efficient. Like any machine, internal combustion engines waste a lot of their theoretical performance—less than 30 percent of the energy in each drop of gas is used to actually move the car. That's because engines are complicated machines with lots of moving pieces of metal, and moving them around thousands of times a minute generates waste heat. Sucking air into a cylinder at a lower engine speed is also harder than doing it at 6000 rpm. Accessories like the water pump and alternator all suck away some power, too.

Careful design work on these bits won't double power or efficiency, but lots of small improvements here and there add up. More significant gains can most easily be achieved by getting more air into the cylinder, so it's here that many of the world's largest car makers have focused their R&D efforts, taking advantage of the breathtaking increase in computing power to build engines that would be unthinkable even a couple of decades ago. The tech has already begun to pay real dividends to drivers. Here's how it works.

Ford's 1.6 L four-cylinder EcoBoost engine.

Ford Motor Company

Take it from the top

As the name suggests, the internal combustion engine—unlike a steam engine—burns its fuel inside the engine housing. The process has four basic stages. It starts with a combustion chamber capped at one end by intake and exhaust valves, then by a piston, connected to a crankshaft, at the other. As the crankshaft turns, the piston drops to the bottom of the cylinder, sucking in air from the intake valve (this is called the "intake stroke"). At the same time, fuel is sprayed in to the expanding cylinder, which mixes with the air. As the crankshaft continues to rotate, the piston moves back up and compresses this fuel and air mix (the "compression stroke"). Once the piston is at the top of the cylinder and the fuel-air mix is under the greatest pressure, it's ignited by a spark. This causes it to explode, which in turn pushes the piston back down again, turning the crankshaft ("the power stroke"). Finally, as the piston moves back up once more, the exhaust valves open and exit, stage right ("the exhaust stroke").

The flow of air into the engine is controlled by a throttle valve, sitting between the air intake and the engine manifold. Pushing on the right-most pedal in your car opens this valve further. Sensors measure the amount of air coming in, as well as how hot it is. Based on this information, the engine knows how much fuel to add and the spark plugs know when to fire. More air means more fuel, which together mean larger explosions. Larger explosions have more energy to transfer to the pistons, which turn the crankshaft more rapidly.

In a conventional engine, the intake and exhaust valves are controlled by a camshaft, which is turned by the crankshaft (which in turn rotates as the pistons force it down during each bang). Lobes on the cam push each valve open and then allow it to close, timed for each stroke of the cycle. The timing of valves opening and closing is fixed by the shape of the lobes on the camshaft. The amount the valve opens is fixed, too (this is called valve lift). The simplest engines have one intake and one exhaust valve per cylinder, both controlled by a single camshaft. By using two camshafts—one for intake valves and one for exhaust valves—each cylinder can have two or even three intake valves and two exhaust valves, making it easier to get air into and out of the engine. The aim is to get as much air as possible, sometimes expressed as volumetric efficiency. For instance: a two liter engine that sucks in two liters of air each cycle would have a volumetric efficiency of 100 percent. In practice, most engines have a much lower figure.

The Ford F-150 EcoBoost engine being put through a torture test.

Ford Motor Company

Car engines have to operate under lots of different conditions—at idle, under partial load, at full throttle—and we expect them to perform well, no matter what. Engineers have to design the engine to perform well in each scenario, but this comes at the cost of being optimal at none. Building a fixed timing engine that idles well gives up some top end performance; getting good top end performance means giving up some fuel efficiency at lower revs (not to mention more emissions).

When the throttle is fully open at high revs, the goal is to produce as much power as possible. To maximize the amount of air in the cylinder, the intake valves should be open as long as possible even before the start of the intake stroke, then remain open into the compression stroke. Opening the valve before the intake stroke allows it to begin filling the cylinder even as it starts increasing in volume. And, as the mass of air is pulled in by the partial vacuum, it gains momentum that doesn't disappear instantly just because the next stroke has started or because a valve is trying to close against it.

Similarly, it's a good idea to open the exhaust valves a little before the exhaust stroke starts. It requires effort to move the piston up during the exhaust stroke, and opening the exhaust valve a little before the start of the exhaust stroke reduces the pressure the piston has to work against. Ultimately, this decreases frictional losses in the engine. And, as with the air coming into the engine during intake, the exhaust gases have momentum that won't disappear when the piston begins its next stroke (so the exhaust valves can be open for part of the intake stroke).

As a result, engines have an overlap period when both intake and exhaust valves are open. For high performance, a longer overlap is beneficial, but at low engine speed the pistons aren't moving as quickly, the partial vacuum generated by the expanding cylinder volume is lower, and so the velocity of the air being sucked into the cylinder is lower. At low engine speed, that means a long overlap can allow exhaust gases to enter the manifold and unburned fuel to exit the exhaust valves. This is why highly tuned engines like those in race cars don't like idling very much.

A cutaway of an engine showing the intake (left) and exhaust (right) valves.

Two things: the "legion of internet memes" link goes to http://arstechnica.com/features/2012/10/, which - as far as I can tell - contains nothing relating to memes or VTEC engines, except insofar as it recurses back to the feature which features the link.

That aside, this is a great article; I can't wait for the next installment.

"Like any machine, internal combustion engines waste a lot of their theoretical performance—less than 30 percent of the energy in each drop of gas is used to actually move the car. That's because engines are complicated machines with lots of moving pieces of metal, and moving them around thousands of times a minute generates waste heat."

You're discussing the mechanical efficiency of the engine in this context. However, an equally large reason most of the internal energy of the fuel is not converted into the kinetic energy of the car is the thermodynamic efficiency of the overall system. If we idealize the combustion products as a high temperature source and the world as a low temperature sink then there is some maximum efficiency that one can extract from a heat engine between those temps (the Carnot efficiency). Increasing the temperature difference between the two makes that efficiency greater. The net effect on air-breathing engines is that the higher the compression ratio the higher the theoretical efficiency. However, cars do not operate with a Carnot cycle engine. The Otto cycle (in automobiles) and the Diesel cycle (in ... diesels) are, by definition lower efficiency than Carnot (with diesels being slightly higher).

So, even in the absence of the mechanical losses you list you've still got the inefficiency of operating as a heat engine (Carnot) and a further reduction based on a non-ideal cycle (Otto or Diesel). Add those up and you're doing well to get half your chemical energy of your fuel converted into motion of your car - even before mechanical losses.

At low engine speed, that means a long overlap can allow exhaust gases to enter the manifold and unburned fuel to exit the exhaust valves. This is why highly tuned engines like those in race cars don't like idling very much.

Well what do you know. At 49 years old and having spent many an adolescent year stripping and rebuilding engines (and a well funded middle age cursing the poor idling of a P-51) I have learnt something new. Thanks very much.

At low engine speed, that means a long overlap can allow exhaust gases to enter the manifold and unburned fuel to exit the exhaust valves. This is why highly tuned engines like those in race cars don't like idling very much.

Well what do you know. At 49 years old and having spent many an adolescent year stripping and rebuilding engines (and a well funded middle age cursing the poor idling of a P-51) I have learnt something new. Thanks very much.

this is what causes that "lopey idle" of some really cammed-up musclecars. That long duration/high overlap just kills manifold vacuum.

"Ford's EcoBoost engines combine turbochargers with direct injection of fuel into the cylinders, something Audi has pioneered after perfecting the system on their Le Mans-winning R8 race cars in the early 2000s."

Idunno about "perfecting" the system, but VW has been selling turbo-charged direct injection diesel engines in the mass market since 1996 (in the US). Nice to see US manufacturers finally catching up with 15 year old diesel engine technology.

"Ford's EcoBoost engines combine turbochargers with direct injection of fuel into the cylinders, something Audi has pioneered after perfecting the system on their Le Mans-winning R8 race cars in the early 2000s."

Idunno about "perfecting" the system, but VW has been selling turbo-charged direct injection diesel engines in the mass market since 1996 (in the US). Nice to US manufacturers finally catching up with 15 year old diesel engine technology.

Diesel direct injection is fundamentally different from gasoline direct injection.

"Ford's EcoBoost engines combine turbochargers with direct injection of fuel into the cylinders, something Audi has pioneered after perfecting the system on their Le Mans-winning R8 race cars in the early 2000s."

Idunno about "perfecting" the system, but VW has been selling turbo-charged direct injection diesel engines in the mass market since 1996 (in the US). Nice to see US manufacturers finally catching up with 15 year old diesel engine technology.

[edit: fix typo]

diesels have been direct injection and turbocharged for decades. VW didn't invent it. The 7.3 liter Powerstroke engine Ford introduced in 1994 was DI/Turbo, medium/heavy truck engines have been DI/turbo since like forever.

The flip side of these wonderfully-machined engines is the time and expense required to maintain them. Increasingly, mechanics are unable to maintain and/or troubleshoot manufacturer's engines without expensive diagnostic tools and a thorough background knowledge of them. On the plus side, because of the remarkable degree of computer control in an engine, a major engine issue can be solved with keystrokes.

In his book Shop Class as Soulcraft, Matthew Crawford looks at the idea that the increasing move to take control away from those willing to investigate their vehicles, and putting it in the hands of computers, has resulted in fewer people interested in the mechanical gubbins of a car. Indeed, in most cars there is a plastic cover over the engine, a hood underneath the hood, designed to make the car look good, but also to discourage an understanding of what makes things happen in an engine. The increasing move towards computerization in engines is fantastic - More fuel efficiency, greater reliability, and easier to use are all immediate benefits. The drawbacks of not being able to fix your own car, or indeed, not having an atmosphere to pick it apart and figure out how it works without thousands of dollars in electronic equipment... Well, that's not so great, and perhaps part of the reason fewer and fewer people are moving into the trades.

..., in most cars there is a plastic cover over the engine, a hood underneath the hood, designed to make the car look good, but also to discourage an understanding of what makes things happen in an engine.

You forgot about the benifit to airflow and cooling that the plastic cover is really designed for...

Nwambe wrote:

... and perhaps part of the reason fewer and fewer people are moving into the trades.

I would hazard that the crap wages and working conditions of an auto mechanic vs. other skilled trades might have something to do with the lack of good and experianced mechanics.

Ok I'm confused, I'll admit that I don't know much about internal combustion engines, but "sucking air into a cylinder at a lower engine speed is also harder than doing it at 6000 rpm" is just completely counter-intuitive to me.

The flip side of these wonderfully-machined engines is the time and expense required to maintain them. Increasingly, mechanics are unable to maintain and/or troubleshoot manufacturer's engines without expensive diagnostic tools and a thorough background knowledge of them. On the plus side, because of the remarkable degree of computer control in an engine, a major engine issue can be solved with keystrokes.

In his book Shop Class as Soulcraft, Matthew Crawford looks at the idea that the increasing move to take control away from those willing to investigate their vehicles, and putting it in the hands of computers, has resulted in fewer people interested in the mechanical gubbins of a car. Indeed, in most cars there is a plastic cover over the engine, a hood underneath the hood, designed to make the car look good, but also to discourage an understanding of what makes things happen in an engine. The increasing move towards computerization in engines is fantastic - More fuel efficiency, greater reliability, and easier to use are all immediate benefits. The drawbacks of not being able to fix your own car, or indeed, not having an atmosphere to pick it apart and figure out how it works without thousands of dollars in electronic equipment... Well, that's not so great, and perhaps part of the reason fewer and fewer people are moving into the trades.

You can definitely still pick apart a lot of modern engines and computer control systems. It just takes a level of technical proficiency that most mechanics would lack.

People have been delving into the machine code of engine computers since the engine computer was introduced. You need to be a new breed of grease monkey. The kind that can hack machine code as well as turn a wrench. They do exist.

That is not to say that some of the systems aren't closed off by way of forms of DRM, of course. That is a shame, and I suspect more of these ECUs will be closed off in the future.

At low engine speed, that means a long overlap can allow exhaust gases to enter the manifold and unburned fuel to exit the exhaust valves. This is why highly tuned engines like those in race cars don't like idling very much.

Well what do you know. At 49 years old and having spent many an adolescent year stripping and rebuilding engines (and a well funded middle age cursing the poor idling of a P-51) I have learnt something new. Thanks very much.

this is what causes that "lopey idle" of some really cammed-up musclecars. That long duration/high overlap just kills manifold vacuum.

Thanks for the links! I drive a supercharged Jag XF (fuel is 16c per Litre here in Dubai .. and my company pays for it anyway) , my next door neighbour has one of those Mustangs and it sounds gorgeous.

At low engine speed, that means a long overlap can allow exhaust gases to enter the manifold and unburned fuel to exit the exhaust valves. This is why highly tuned engines like those in race cars don't like idling very much.

Well what do you know. At 49 years old and having spent many an adolescent year stripping and rebuilding engines (and a well funded middle age cursing the poor idling of a P-51) I have learnt something new. Thanks very much.

this is what causes that "lopey idle" of some really cammed-up musclecars. That long duration/high overlap just kills manifold vacuum.

"Ford's EcoBoost engines combine turbochargers with direct injection of fuel into the cylinders, something Audi has pioneered after perfecting the system on their Le Mans-winning R8 race cars in the early 2000s."

Idunno about "perfecting" the system, but VW has been selling turbo-charged direct injection diesel engines in the mass market since 1996 (in the US). Nice to US manufacturers finally catching up with 15 year old diesel engine technology.

Diesel direct injection is fundamentally different from gasoline direct injection.

To clarify, diesel engines utilize compression ignition (no sparkplugs), so direct injection is simply a matter of synchronizing with the engine timing. A direct-injection diesel is a very simple, reliable engine, as exemplified in its use in a wide variety of military vehicles including the HMMWV (Hummer), which debuted in 1984.

Gasoline direct injection is more difficult, requiring synchronization between timing (engine rotation / valve opening) AND sparkplug ignition. As the article mentions, this exciting technology will benefit us gear-heads greatly in reduced fuel costs as it is rolled out across the fleet - great article!

Ok I'm confused, I'll admit that I don't know much about internal combustion engines, but "sucking air into a cylinder at a lower engine speed is also harder than doing it at 6000 rpm" is just completely counter-intuitive to me.

At 6000 rpm, the pistons are moving much more rapidly, which means the cylinders are expanding (and contracting) in volume much more rapidly. As the cylinder increases in volume faster, there's a more rapid change in pressure between the manifold (high pressure) and the cylinder (low pressure) so it sucks air in more quickly. Does that make sense?

"Ford's EcoBoost engines combine turbochargers with direct injection of fuel into the cylinders, something Audi has pioneered after perfecting the system on their Le Mans-winning R8 race cars in the early 2000s."

This is incorrect.

The first of the modern direct-injected turbo-charged car was back in 1996 with Mitsubishi's GDI.

Audi was actually a late player in the direct-injection market; Renault, Nissan, Toyota all brought out direct-injected cars in the late-90s. There obviously was "direct-injected" cars even before that, notably in the 300SL Gullwing, but they aren't anything like what direct-injection is today.

Back in 1996 when Mitsubishi released GDI both Europe and the US used high-sulfur fuel. Its only with the availability of "Ultra low Sulfur Gasoline" that DI engines became widely feasible. So a lot of the technology we see in engines is a direct result of better petroleum processing.

The final goal of direct-injection, and I'm sad its not mentioned here, is HCCI. Basically, the Otto-Diesel engine, "80% the efficiency of diesel with 20% the cost". Basically, diesel-like compression ratios that don't require a spark-plug.

The intermediate between DI and HCCI is already here. With 14:1 compression ratio that is in Mazda's new SkyActiv technology which is near diesel level.s

But again, the problem is gasoline. For the US, the engine needs to be tuned down to 13:1 due to the quality of US unleaded. Europe and Japan will keep the 14:1 compression ratio. But soon we'll have near HCCI level of fuel efficiency.

Ok I'm confused, I'll admit that I don't know much about internal combustion engines, but "sucking air into a cylinder at a lower engine speed is also harder than doing it at 6000 rpm" is just completely counter-intuitive to me.

Your (non-diesel) car must operate at a specific air-fuel ratio very near what's termed stoichiometric. The amount of energy that's pulled into the cylinder on any given stroke is proportional to the mass of the fuel. So when I want low power I can slow my revs down so I have fewer intakes per second. However, that's generally not enough of a change to get from idle to max power.

Instead, your throttle restricts the air flow into the cylinders. At max power the throttle is basically wide open allowing as much air in as the aerodynamics will allow. At anything less than max power the throttle reduces the amount of air sucked into the engine and hence lowers the amount of fuel drawn in as well. Generally at 6k RPM you're at max throttle but if you're downshifting or going downhill it is possible that the throttle may not be engaged at all and you're actually not free breathing. It's the fact that you're at full throttle is easier to breathe not that there's anything special about 6k RPM.

Unfortunately, full power is generally a very small amount of time for many drivers. Ideally you'd like to drive down the highway at full throttle but at low RPMs. Car makers get closer to this by squeezing mroe and more power out of smaller and smaller displacement engines so that the car does not feel unresponsive when you actually want the power (which is the rest of the article here).

I'd love to see an analysis along these lines of the new Formula 1 2014 spec engine. After all the goal is to make a turbo charge 1.6T engine that produces something like 700-800hp.

The F1 engines in the mid-80s turbo were only 1.5L but could put out 600+ HP in race spec and were rumored to be nearer 1000 for qualifying (where they basically only had to last for 3 laps.) The main difference between then and the 2014 spec is in fuel efficiency and durability.

Ok I'm confused, I'll admit that I don't know much about internal combustion engines, but "sucking air into a cylinder at a lower engine speed is also harder than doing it at 6000 rpm" is just completely counter-intuitive to me.

At 6000 rpm, the pistons are moving much more rapidly, which means the cylinders are expanding (and contracting) in volume much more rapidly. As the cylinder increases in volume faster, there's a more rapid change in pressure between the manifold (high pressure) and the cylinder (low pressure) so it sucks air in more quickly. Does that make sense?

Yes it'll move faster, I'm with you so far, but what it seems like to me is that at low rpm, the volumetric efficiency can be higher because the pressure differential has more time to disappear, whereas at high rpm the piston has to actually pull the air in.. oh well I guess I just don't get it, back to school for me

edit: ok nevermind - I forgot about the throttle. Somehow I was just thinking low rpm without throttling. Oh well, mea culpa.

Mazada's Skyactive does 14:1. How, i do not know. But I do know CRs have increased from 8 to 11 over time. The 14:1 is more efficient (15%)

I drive a S54 engine (BMW 3.2L - 330hp) It was the first production engine to break 100HP/L. Then Honda got the S2000 engine up to 120HP/L. How did they do this? Ridiculous parts counts. In addition to the direct injection, the 6 cyl engine has 6 individually controlled throttle bodies.

Once the piston is at the top of the cylinder and the fuel-air mix is under the greatest pressure, it's ignited by a spark. This causes it to explode, which in turn pushes the piston back down again, turning the crankshaft ("the power stroke").

It's really important to note that the spark causes the fuel/air mixture to burn in a properly tuned engine, not explode. Explosion of the fuel (also called knock) is dangerous and will lead to engine failure (often catastrophic) so it's important to note this difference. The article refers to knock as pre-detonation, but I don't think that's very accurate.

Ok I'm confused, I'll admit that I don't know much about internal combustion engines, but "sucking air into a cylinder at a lower engine speed is also harder than doing it at 6000 rpm" is just completely counter-intuitive to me.

At 6000 rpm, the pistons are moving much more rapidly, which means the cylinders are expanding (and contracting) in volume much more rapidly. As the cylinder increases in volume faster, there's a more rapid change in pressure between the manifold (high pressure) and the cylinder (low pressure) so it sucks air in more quickly. Does that make sense?

Not sure I'd agree with that explanation. However, I did also overlook one more detail in my post above: acoustic tuning for greater than 100% volumetric efficiency.

Every time you open and/or close a valve there is a reflected compression wave or an expansion wave that will propagate down the intake/exhaust piping. On the exhaust side you're dealing with quite high pressures so these can be reall strong (and why you have a muffler on your exhaust and not on your intake). Every time one of these waves hits a section of the manifold where two pipes meet there will be a reflection. If you're really clever you can "tune" your manifold so that these reflections result in a pressure wave arriving on your intake valves as they open or an expansion wave as your exhaust valves open. Working together this will squish more air in and suck more air out.

You can optimize so that at one particular RPM you've got a really massive volumetric efficiency and therefore power. However, cars for consumers are tuned across a wide RPM/throttle range so that you end up with higher power everywhere. However, this effect is more pronounced at the higher RPM range. Since things happen "slowly" at low RPMs you'd need very long manifold piping to get the most advantage. Therefore modern card "breathe" better at higher RPMs.

How much of the engine loss is due to accelerating and decelerating the mass of the cylinder head and connecting rods 6,000 times a second? It seems to me that the Wankle engine had some significant advantages in potential efficiency that were obliterated by what would reasonably be considered falsified fuel economy numbers published in the late 1970s by the US government.

Of course there were some real issues with the mechanical design of the early versions of these engines, particularly in the area of combustion chamber seals, but my impression is that most of those were resolved relatively quickly in the first few production years. The fuel economy numbers were the real death knell of the Wankle engine.